Bug#14463247 ORDER BY SUBQUERY REFERENCING OUTER ALIAS FAILS

Documentation for class Item_outer_ref was wrong:
(*ref) may point to Item_field as well
(see e.g. Item_outer_ref::fix_fields)

So this casting in get_store_key() was wrong:
(*(Item_ref**)((Item_ref*)keyuse->val)->ref)->ref_type()

sql/item.h

@@ -2676,6 +2676,7 @@ class Item_direct_view_ref :public Item_direct_ref
   resolved is a grouping one. After it has been fixed the ref field will point
   to either an Item_ref or an Item_direct_ref object which will be used to
   access the field.
+  The ref field may also point to an Item_field instance.
   See also comments for the fix_inner_refs() and the
   Item_field::fix_outer_field() functions.
 */

sql/sql_select.cc

@@ -6019,19 +6019,33 @@ get_store_key(THD *thd, KEYUSE *keyuse, table_map used_tables,
 				    key_part->length,
 				    keyuse->val);
   }
-  else if (keyuse->val->type() == Item::FIELD_ITEM ||
-           (keyuse->val->type() == Item::REF_ITEM &&
-            ((Item_ref*)keyuse->val)->ref_type() == Item_ref::OUTER_REF &&
-            (*(Item_ref**)((Item_ref*)keyuse->val)->ref)->ref_type() ==
-             Item_ref::DIRECT_REF && 
-            keyuse->val->real_item()->type() == Item::FIELD_ITEM))
+
+  Item_field *field_item= NULL;
+  if (keyuse->val->type() == Item::FIELD_ITEM)  
+    field_item= static_cast<Item_field*>(keyuse->val->real_item());
+  else if (keyuse->val->type() == Item::REF_ITEM)
+  {
+    Item_ref *item_ref= static_cast<Item_ref*>(keyuse->val);
+    if (item_ref->ref_type() == Item_ref::OUTER_REF)
+    {
+      if ((*item_ref->ref)->type() == Item::FIELD_ITEM)
+        field_item= static_cast<Item_field*>(item_ref->real_item());
+      else if ((*(Item_ref**)(item_ref)->ref)->ref_type()
+               == Item_ref::DIRECT_REF
+               && 
+               item_ref->real_item()->type() == Item::FIELD_ITEM)
+        field_item= static_cast<Item_field*>(item_ref->real_item());
+    }
+  }
+  if (field_item)
     return new store_key_field(thd,
-			       key_part->field,
-			       key_buff + maybe_null,
-			       maybe_null ? key_buff : 0,
-			       key_part->length,
-			       ((Item_field*) keyuse->val->real_item())->field,
-			       keyuse->val->full_name());
+                               key_part->field,
+                               key_buff + maybe_null,
+                               maybe_null ? key_buff : 0,
+                               key_part->length,
+                               field_item->field,
+                               keyuse->val->full_name());
+
   return new store_key_item(thd,
 			    key_part->field,
 			    key_buff + maybe_null,